Fuel or fuel additive containing doped cerium oxide nanoparticles

ABSTRACT

A fuel or fuel additives is disclosed which includes particles of cerium oxide which have been doped with a divalent or trivalent metal or metalloid which is a rare earth metal, a transition metal or a metal of group IIA, IIIB, VB, or VIB of the Periodic Table.

RELATED APPLICATIONS

This application is a national stage filing under 35 U.S.C. § 371 of PCTInternational application PCT/GB02/05013, filed Nov. 6, 2002, which waspublished under PCT Article 21(2) in English.

This invention relates to cerium oxide nanoparticles which are useful ascatalysts.

Cerium oxide is widely used as a catalyst in three way converters forthe elimination of toxic exhaust emission gases in automobiles. Theceria contained within the catalyst can act as a chemically activecomponent, working as an oxygen store by release of oxygen in thepresence of reductive gases, and removal of oxygen by interaction withoxidising species.

Cerium oxide may store and release oxygen by the following processes:2CeO₂⇄Ce₂O₃+½O₂

The key to the use of ceria for catalytic purposes is the low redoxpotential between the Ce³⁺ and Ce⁴⁺ ions (1.7V) that allows the abovereaction to easily occur in exhaust gases. Cerium oxide may provideoxygen for the oxidation of CO or C_(n)H_(n) or may absorb oxygen forthe reduction of NO_(x). The amounts of oxygen reversibly provided inand removed from the gas phase are called the oxygen storage capacity(OSC) of ceria.

The above catalytic activity may occur when cerium oxide is added as anadditive to fuel, for example diesel or petrol. However, in order forthis effect to be useful the cerium oxide must be of a particle sizesmall enough to remain in a stable dispersion in the fuel. The ceriumoxide particles must be of a nanocrystalline nature, for example theyshould be less than 1 micron in size, and preferentially 1–300 nm insize. In addition, as catalytic effects are surface area dependant thesmall particle size renders the nanocrystalline material more effectiveas a catalyst.

It has now been found, according to the present invention, that thecatalytic efficiency of cerium oxide can be enhanced by addition offurther components in the material. In particular it has been found thatcerium oxide may be doped with components that result in additionaloxygen vacancies being formed. Thus doping will generally besubstitution doping as opposed to interstitial doping. This will clearlyenhance the OSC of the material, and hence its catalytic properties.Such dopant ions must be di- or tri-valent in order to provide oxygenvacancies. They must also be of a size that allows incorporation of theion within the surface region of the cerium oxide nanoparticles.Accordingly metals with a large ionic radius should not be used. Forexample transition metals in the first and second row of transitionmetals are generally preferred over those listed in the third. The ceriaserves as the oxygen activation and exchange medium during a redoxreaction. However, because ceria and the like are ceramic materials,they have low electronic conductivity and low activity surface sites forthe chemisorption of the reacting species. Transition metal additivesare particularly useful to improve this situation. In addition,multivalent dopants will also have a catalytic effect of their own.

It is believed that doping in this way changes the zeta potential andthus improves the dispersion.

According to the present invention there is provided a fuel additivewhich comprises a particle of cerium oxide which has been doped with adivalent or trivalent metal or metalloid which is a rare earth metal, atransition metal, including a noble metal, or a metal of Group IIA,IIIB, VB, or VIB of the Periodic Table and a polar or non-polar organicsolvent as well as a fuel containing such an additive or such particles.Typically the oxides will have the formula Ce_(1−x)M_(x)O₂ where M is asaid metal or metalloid, in particular Rh, Cu, Ag, Au, Pd, Pt, Sb, Se,Fe, Ga, Mg, Mn, Cr, Be, B, Co, V and Ca as well as Pr, Sm and Gd and xhas a value up to 0.3, typically 0.01 or 0.1 to 0.2, or of the formula[(CeO₂)_(1−n)(REO_(y))_(n)]_(1−k)M′_(k) where M′_(k) is a said metal ormetalloid other than a rare earth, RE is a rare earth y is 1 or 1.5 andeach of n and k, which may be the same or different, has a value up to0.5, preferably up to 0.3, typically 0.01 or 0.1 to 0.2. Copper isparticularly preferred. If too much dopant is used, there will be anincreasing tendency for it to form an oxyanion thus negating thebenefits of introducing it.

In general the particles will have a size not exceeding 1 micron andespecially not exceeding 300 nm, for example 1 to 300 nm, such asbetween 5 and 150 nm, in particular 10 to 50 nm, especially 10 to 20 nm.

Dopants may be incorporated into the cerium oxide nanoparticlesprincipally by one of the following:

i) Doping within the particle during formation, e.g. byco-precipitation.

ii) Absorption of dopant ions onto the surface followed by firing of thedopant ion into the material. Note that particle size of the ceriumoxide does not increase during firing.

(iii) A combustion synthesis. Doping during formation can be achieved bya combustion process whereby a mixture of salts of cerium and the dopantmetal is heated together with, for example, glycine or other combustiblesolvent, preferably oxygen-containing, such as aliphatic alcohols, forexample C_(1–C) ₆ alcohols, in particular isopropyl alcohol, in a flameto convert it to the oxide.

(iv) A mechano-chemical process typically involving milling, generallyusing a ball mill such as that described in WO99/59754 which involvessubjecting a cerium oxide precursor and a dopant precursor in anon-reactive diluent to mechanical milling, heat treating the resultingmaterial to convert it into the oxide and removing the diluent. Theprecursors are typically hydroxides, carbonates, sulphates oroxychlorides, especially cerium hydroxide and cerium carbonate. Atypical diluent is sodium chloride which can readily be removed withwater.

(v) A double decomposition process whereby, for example a salt of ceriumand of the dopant, such as nitrate or chloride is reacted with a solubleoxide or hydroxide, for example of magnesium or calcium and theresulting oxide or hydroxide is recovered and the water soluble removed,typically by washing. In the case of the hydroxide, this is fired toconvert it to the desired doped oxide.

Although it is clear that techniques other than (ii) will result in adopant distribution that is even within the particle and the second mayresult in a predominately surface doping this is of little importancesince the reaction involves surface based catalysis. The relativeconcentrations of dopant for optimum performance will vary however.

Of particular importance is the doping of copper into the cerium oxidenanoparticles. The redox properties of the copper-cerium system havebeen recognised as being synergistic, with the combination being morereadily reduced than the corresponding independent compounds.

Doping during formation typically involves mixing, in an aqueoussolution, a water-soluble cerium salt and a water-soluble salt of thedopant and raising the pH of the solution to cause the desired dopedcerium oxide to precipitate.

Suitable salts include nitrates and carbonates. The pH can be raised bythe addition of an alkali such as ammonium hydroxide. A final pHexceeding 8, typically 8 to 10, is generally needed.

As indicated, the dopant can also be inserted by firing. The dopant ioncan be incorporated into the host lattice of cerium oxide by a bakingtechnique typically at from 600° C. to 1000° C. For this purpose ceriumoxide and a salt of the metal dopant can be mixed in water and, ifdesired, ultrasonicated for, say, 10 minutes and boiled dry. Thematerial is then fired, typically for several hours, for example 3hours, to give the doped material.

It will be appreciated that although reference is made to doping with aspecific metal or metalloid, the metal or metalloid can be introduced asan oxide or, initially, a salt which is converted into an oxide duringthe process.

It will also be appreciated that, if desired, more than one dopant canbe used. Likewise the cerium oxide can be in the form of a mixed oxidei.e. another tetravalent metal can be incorporated such as zirconium (ordoped with both a rare earth and another metal or metalloid M′).

The amount of dopant incorporated can, of course, be adjusted bycontrolling the amount of doping salt employed, as one skilled in theart will appreciate.

It is preferred that the particles are coated to prevent agglomeration.For this purpose the particles can be comminuted in an organic solventin the presence of a coating agent which is an organic acid, anhydrideor ester or a Lewis base. It has been found that, in this way whichinvolves coating in situ, it is possible to significantly improve thecoating of the oxide. Further, the resulting product can, in manyinstances, be used directly without any intermediate step. Thus in somecoating procedures it is necessary to dry the coated particles beforedispersing them in a hydrocarbon solvent.

Thus the cerium oxide can be dispersible or soluble in the (liquid) fuelor another hydrocarbon or other solvent compatible with the fuel.

The particles which are subjected to the process should have as large asurface area as possible and preferably the particles have a surfacearea, before coating, of at least 10 m²/g and preferably a surface areaof at least 50 or 75 m²/g, for example 80–150 m²/g, or 100–300 m²/g.

The coating agent is suitably an organic acid, anhydride or ester or aLewis base. The coating agent is preferably an organic carboxylic acidor an anhydride, typically one possessing at least 8 carbon atoms, forexample 10 to 25 carbon atoms, especially 12 to 18 carbon atoms such asstearic acid. It will be appreciated that the carbon chain can besaturated or unsaturated, for example ethylenically unsaturated as inoleic acid. Similar comments apply to the anhydrides which can be used.A preferred anhydride is dodecylsuccinic anhydride. Other organic acids,anhydrides and esters which can be used in the process of the presentinvention include those derived from phosphoric acid and sulphonic acid.The esters are typically aliphatic esters, for example alkyl esterswhere both the acid and ester parts have 4 to 18 carbon atoms.

Other coating or capping agents which can be used include Lewis baseswhich possess an aliphatic chain of at least 8 carbon atoms includingmercapto compounds, phosphines, phosphine oxides and amines as well aslong chain ethers, diols, esters and aldehydes. Polymeric materialsincluding dendrimers can also be used provided that they possess ahydrophobic chain of at least 8 carbon atoms and one or more Lewis basegroups, as well as mixtures of two or more such acids and/or Lewisbases.

Typical polar Lewis bases include trialkylphosphine oxides P(R³)₃O,especially trioctylphosphine oxide (TOPO), trialkylphosphines, P(R³)₃,amines N(R³)₂, thiocompounds S(R³)₂ and carboxylic acids or estersR³COOR⁴ and mixtures thereof, wherein each R³, which may be identical ordifferent, is selected from C₁₋₂₄ alkyl groups, C₂₋₂₄ alkenyl groups,alkoxy groups of formula —O(C₁₋₂₄ alkyl), aryl groups and heterocyclicgroups, with the proviso that at least one group R³ in each molecule isother than hydrogen; and wherein R⁴ is selected from hydrogen and C₁₋₂₄alkyl groups, preferably hydrogen and C₁₋₁₄ alkyl groups. Typicalexamples of C₁₋₂₄ and C₁₋₄ alkyl groups, C₂₋₂₄ alkenyl groups, arylgroups and heterocyclic groups are described below.

It is also possible to use as the polar Lewis base a polymer, includingdendrimers, containing an electron rich group such as a polymercontaining one or more of the moieties P(R³)₃O, P(R³)₃, N(R³)₂, S(R³)₂or R³COOR⁴ wherein R³ and R⁴ are as defined above; or a mixture of Lewisbases such as a mixture of two or more of the compounds or polymersmentioned above.

As used herein, a C₁₋₄ alkyl group is an alkyl group as defined abovewhich contains from 1 to 4 carbon atoms. C₁₋₄ alkyl groups includemethyl, ethyl, i-propyl, n-propyl, n-butyl and tert-butyl.

As used herein, a C₂₋₂₄ alkenyl group is a linear or branched alkenylgroup which may be unsubstituted or substituted at any position andwhich may contain heteroatoms selected from P, N, O and S. Typically, itis unsubstituted or carries one or two substituents. Suitablesubstituents include halogen, hydroxyl, cyano, —NR₂, nitro, oxo, —CO₂R,—SOR and —SO₂R wherein each R may be identical or different and isselected from hydrogen or C₁₋₄ alkyl.

As used herein, a C₂₋₄ alkenyl group is an alkenyl group as definedabove which contains from 2 to 4 carbon atoms. C₂₋₄ alkenyl groupsinclude ethenyl, propenyl and butenyl.

As used herein, an aryl group is typically a C₆₋₁₀ aryl group such asphenyl or naphthyl, preferably phenyl. An aryl group may beunsubstituted or substituted at any position, with one or moresubstituent. Typically, it is unsubstituted or carries one or twosubstituent. Suitable substituent include C₁₋₄ alkyl, C₁₋₄ alkenyl, eachof which may be substituted by one or more halogens, halogen, hydroxyl,cyano, —NR₂, nitro, oxo, —CO₂R, —SOR and —SO₂R wherein each R may beidentical or different and is selected from hydrogen and C₁₋₄ alkyl.

As used herein, a heterocyclic group is a 5- to 10-membered ringcontaining one or more heteroatoms selected from N, O and S. Typicalexamples include pyridyl, pyrazinyl, pyrimidinyl, pyridazinyl, furanyl,thienyl, pyrazolidinyl, pyrrolyl and pyrazolyl groups. A heterocyclicgroup may be substituted or unsubstituted at any position, with one ormore substituent. Typically, a heterocyclic group is unsubstituted orsubstituted by one or two substituents. Suitable substituents includeC₁₋₄ alkyl, C₁₋₄ alkenyl, each of which may be substituted by one ormore halogens, halogen, hydroxyl, cyano, —NR₂, nitro, oxo, —CO₂R, —SORand —SO₂R wherein each R may be identical or different and is selectedfrom hydrogen and C₁₋₄ alkyl.

As used herein, halogen is fluorine, chlorine, bromine or iodine,preferably fluorine, chlorine or bromine.

The coating process can be carried out in an organic solvent.Preferably, the solvent is non-polar and is also preferablynon-hydrophilic. It can be an aliphatic or an aromatic solvent. Typicalexamples include toluene, xylene, petrol, diesel fuel as well as heavierfuel oils. Naturally, the organic solvent used should be selected sothat it is compatible with the intended end use of the coated particles.The presence of water should be avoided; the use of an anhydride ascoating agent helps to eliminate any water present.

The coating process involves comminuting the particles so as to preventany agglomerates from forming. The technique employed should be chosenso that the particles are adequately wetted by the agent and a degree ofpressure or shear is desirable. Techniques which can be used for thispurpose include high-speed stirring (e.g. at least 500 rpm) or tumbling,the use of a colloid mill, ultrasonics or ball milling. Ball milling ispreferred. Typically, ball milling can be carried out in a pot where thelarger the pot the larger the balls. By way of example, ceramic balls of7 to 10 mm diameter are suitable when the milling takes place in a 1.25liter pot. The time required will of course, be dependent on the natureof the particles but, generally, at least 4 hours is required. Goodresults can generally be obtained after 24 hours so that the typicaltime is 12 to 36 hours.

The effectiveness of the process can be assessed by studying thestability of the resulting suspension. A turbidity procedure can be usedto assess the extent to which the particles remain suspended andtherefore un-agglomerated. The agglomerated particles will, of coursefall out of suspension and therefore reduce the turbidity of thesuspension. By way of example, it has been found that the addition of asuspension of cerium oxide particles obtained by the process issufficient to act as a fuel catalyst when present in a concentration ofabout 4 ppm. This compares with a concentration of in excess of 40 ppmfor an existing coated cerium oxide product.

The incorporation of the cerium oxide in fuel serves more than onepurpose. The primary purpose is to act as a catalyst in the reduction oftoxic exhaust gases on combustion of the fuel. However, it can serveanother purpose in diesel engines. Diesel engines increasingly comprisea trap for particulates resulting from combustion of the diesel fuel.The presence of the cerium oxide in the traps helps to burn off theparticulates which accumulate in the trap. Additionally organo platinumgroup metal compounds can be present as co-catalysts. Thus the fuels ofthe present invention can also comprise such a platinum group metalcompound. These should be soluble in the fuel and include compounds ofplatinum or, for example, palladium and rhodium and mixtures of two ormore thereof.

Suitable compounds include platinum acetylacetonate and compounds havingthe formula: X Pt R₁ R₂ where X is a ligand containing at least oneunsaturated carbon-carbon double bond which can be olefinic, acetylenicor aromatic and R₁ and R₂ are, independently, benzyl, phenyl,nitrobenzyl or alkyl of 1 to 10 carbon atoms such as diphenylcyclooctadiene platinum (II). The use of the doped ceria enables one touse less of the organo platinum group metal than if undoped ceria isused and this represents an economic advantage.

If desired the resulting particles can be dried and re-dispersed inanother organic solvent or in a polymer. Examples of suitable polymersinclude homo- and co-polymers of ethylene, propylene or styrene, andhydrocarbon-based elastomers such as those containing propylene,butadiene or isoprene.

The particles can be incorporated into the fuel directly or as anadditive for the fuel. The particles are preferably incorporated intodiesel fuel.

Typical additives which can be used in the fuel compositions, especiallydiesel fuel, include those conventionally used, such as:

Non polar organic solvents such as aromatic and aliphatic hydrocarbonssuch as toluene, xylene and white spirit, e.g. those sold under theTrade Mark “SHELLSOL” by the Royal Dutch/Shell Group,

Polar organic solvents, in particular, alcohols generally aliphaticalcohols e.g. 2 ethylhexanol, decanol and isotridecanol,

Detergents such as hydrocarbyl-substituted amines and amides, e.g. hydrocarbyl-substituted succinimides, e.g. a polyisobutenyl succinimide,

Dehazers, e.g. alkoxylated phenol formaldehyde polymers such as thosecommercially available as “NALCO” (Trade Mark) 7D07 (ex Nalco), and“TOLAD” (Trade Mark) 2683 (ex Petrolite),

Anti-foaming agents e.g. the polyether-modified polysiloxanescommercially available as “TEGOPREN” (Trade Mark) 5851 (ex Th.Goldschmidt) Q 25907 (ex Dow Corning) or “RHODORSIL” (Trade Mark) (exRhone Poulenc))

Ignition improvers such as aliphatic nitrates e.g. 2-ethylhexyl nitrateand cyclohexyl nitrate,

Anti-rust agents such as polyhydric alcohol esters of succinic acidderivatives (e.g. commercially sold by Rhein Chemie, Mannheim, Germanyas “RC 4801”, or by Ethyl corporation as HiTEC 536),

Reodorants,

Anti-oxidants e.g. phenolics such as 2,6-di-tert-butylphenol, orphenylenediamines such as N,N′-di-sec-butyl-p-phenylenediamine,

Metal deactivators such as salicylic acid derivatives, e.g.N,N¹-disalicylidene-1,2-propane diamine and,

Lubricity agents such as fatty acids and esters, (e.g. thosecommercially available as EC831, P631, P633 or P639 (ex Infinium) or“HITEC” (Trade Mark) 580 (ex Ethyl Corporation), “Lubrizol” (trade mark)539A (ex Lubrizol), “VECTRON” (trade mark) 6010 (ex Shell Additives),OLI9000 (ex Associated Octel),

Unless otherwise stated, the (active matter) concentration of eachadditive in the fuel is generally up to 1000 ppmw (parts per million byweight of the diesel fuel), in particular up to 800 ppmw, e.g. 1 to1000, 1 to 800 or 1–20, ppmw.

The (active matter) concentration of the dehazer in the diesel fuel ispreferably in the range from 1 to 20 ppmw. The (active matter)concentrations of other additives (with the exception of the detergent,ignition improver and the lubricity agent) are each preferably up to 20ppmw. The (active matter) concentration of the detergent is typically upto 800 ppmw e.g. 10 to 500 ppmw. The (active matter) concentration ofthe ignition improver in the diesel fuel is preferably up to 600 ppmwe.g. 100 to 250 ppmw. If a lubricity agent is incorporated into thediesel fuel, it is conveniently used in an amount of 100 to 500 ppmw.

Some of these additives are more commonly added directly at the refinerywhile the others form part of a diesel fuel additive (DFA), typicallyadded at the point of loading with the tanker. A typical DFA comprises:

detergent 10–70% (by weight) antirust  0–10% antifoam  0–10% dehazer 0–10% non-polar solvent  0–50% polar solvent  0–40%

The diesel oil itself may be an additised (additive-containing) oil. Ifthe diesel oil is an additised oil, it will contain minor amounts of oneor more additives, e.g. anti-static agents, pipeline drag reducers, flowimprovers, e.g. ethylene/vinyl acetate copolymers or acrylate/maleicanhydride copolymers, and wax anti-settling agents, e.g. thosecommercially available under the Trade Marks “PARAFLOW” (e.g. “PARAFLOW”450; ex Paramins), “OCTEL” (e.g. “OCTEL” W 5000; ex Octel) and“DODIFLOW” (e.g. “DODIFLOW” V 3958; ex Hoechst).

The same or similar additives can be used for other fuels such aspetrol, as one skilled in the art will appreciate.

The present invention also provides a method of improving the combustionof a fuel which comprises incorporating therein the cerium oxideparticles.

1. A fuel which comprises one or more particles of cerium oxide whichhave been doped with a divalent or trivalent metal or metalloid which isa rare earth metal, a transition metal or a metal of group IIA, IIIB,VB, or VIB of the Periodic Table.
 2. A fuel additive which comprises oneor more particles of cerium oxide which have been doped with a divalentor trivalent metal or metalloid which is a rare earth metal, atransition metal or a metal of group IIA, IIIB, VB, or VIB of thePeriodic Table and a polar or non-polar organic solvent.
 3. The fuelaccording to claim 1 or the fuel additive according to claim 2 whereinthe metal is a transition metal.
 4. The fuel or fuel additive accordingto claim 3 wherein the metal is rhodium, copper, silver, gold,palladium, platinum, iron, manganese, chromium, cobalt or vanadium. 5.The fuel according to claim 1 or the fuel additive according to claim 2wherein the metal is terbium, praeseodymium, samarium, gadolinium,antimony, selenium, gallium, magnesium, beryllium, boron or calcium. 6.The fuel according to claim 1 or the fuel additive according to claim 2wherein the oxide has the formula:Ce_(1−x)M_(x)O₂ where M is said metal or metalloid and x has a value upto 0.3.
 7. The fuel according to claim 1 or the fuel additive accordingto claim 2 wherein the oxide has the formula:[(CeO₂)_(1−n)(REO_(y))_(n)]_(1−k)M′_(k) where M′ is said metal ormetalloid other than a rare earth, RE is a rare earth, y is 1 or 1.5 andeach of n and k, which may be the same or different, has a value up to0.5.
 8. The fuel according to claim 1 or the fuel additive according toclaim 2 wherein the particles have a size not exceeding 1 micron.
 9. Thefuel or fuel additive according to claim 8 wherein the particles have asize from 1 to 300 nm.
 10. The fuel or fuel additive according to claim1 or the fuel additive according to claim 2 wherein the cerium oxide hasbeen doped with more than one said metal or metalloid and/or oxidethereof.
 11. The fuel or fuel additive according to claim 1 or the fueladditive according to claim 2 wherein the particles have been coatedwith an organic acid, anhydride or ester or a Lewis base.
 12. The fuelor fuel additive according to claim 11 in which the coating agent isoleic acid or dodecylsuccinic anhydride.
 13. The fuel according to claim1 which is diesel fuel.
 14. The fuel according to claim 1 which alsocomprises an organo platinum group compound.
 15. The fuel additiveaccording to claim 2 wherein the solvent is an aliphatic or aromatichydrocarbon or an aliphatic alcohol.
 16. The fuel additive according toclaim 2 which comprises one or more of a detergent, dehazer,anti-foaming agent, ignition improver, anti-rust agent, reodorant,anti-oxidant, metal deactivator or lubricity agent.
 17. A method formaking an additive-containing fuel comprising incorporating into a fuelparticles of cerium oxide which have been doped with a divalent ortrivalent metal or metalloid which is a rare earth metal, a transitionmetal or a metal of group IIA, IIIB, VB, or VIB of the Periodic Table.18. A method of improving the combustion of a fuel which comprisesincorporating therein particles of cerium oxide which have been dopedwith a divalent or trivalent metal or metalloid which is a rare earthmetal, a transition metal or a metal of group IIA, IJIB, VB, or VIB ofthe Periodic Table.
 19. A fuel which comprises the fuel additive asclaimed in claim 2.